将一个循环分成两个循环的性能 [英] Performance of breaking apart one loop into two loops

问题描述

美好的一天，

假设您有一个简单的for循环，如下所示...

Suppose that you have a simple for loop like below...

for(int i=0;i<10;i++)
{
    //statement 1
    //statement 2
}

假定语句1和2为O(1).除了启动"另一个循环的少量开销外，将for循环分解为两个(非嵌套但顺序的)循环是否会同样快呢?例如...

Assume that statement 1 and statement 2 were O(1). Besides the small overhead of "starting" another loop, would breaking down that for loop into two (not nested, but sequential) loops be as equally fast? For example...

for(int i=0;i<10;i++)
{
    //statement 1
}
for(int i=0;i<10;i++)
{
    //statement 2
}

为什么我问这样一个愚蠢的问题是我有一个碰撞检测系统(CDS)，该系统必须遍历所有对象.我想将CDS系统的功能分区"，以便我可以简单地调用

Why I ask such a silly question is that I have a Collision Detection System(CDS) that has to loop through all the objects. I want to "compartmentalize" the functionality of my CDS system so I can simply call

cds.update(objectlist);

无需中断cds系统. (不必太担心我的CDS实施...我想我知道我在做什么，我只是不知道如何解释它，我真正需要知道的是我是否会对循环造成巨大的性能损失再次 .

instead of having to break my cds system up. (Don't worry too much about my CDS implementation... I think I know what I am doing, I just don't know how to explain it, what I really need to know is if I take a huge performance hit for looping through all my objects again.

推荐答案

这取决于您的应用程序.

It depends on your application.

可能的缺点(分拆):

• 您的数据不适合L1数据缓存，因此您在第一个循环中加载一次，然后在第二个循环中重新加载

可能的收益(拆分):

• 您的循环包含许多变量，拆分有助于减少寄存器/堆栈压力，并且优化程序将其转换为更好的机器代码
• 您使用的函数会破坏L1指令高速缓存，以便在每次迭代时都加载高速缓存，而通过拆分，您可以设法在每次循环的第一次迭代中仅(一次)加载一次.

这些列表当然并不全面，但是您已经可以感觉到 code 和 data 之间存在紧张关系.因此，当我们都不知道时，很难做出有根据的/疯狂的猜测.

These lists are certainly not comprehensive, but already you can sense that there is a tension between code and data. So it is difficult for us to take an educated/a wild guess when we know neither.

有疑问:个人资料.使用callgrind，在每种情况下检查高速缓存未命中，检查已执行的指令数.测量花费的时间.

In doubt: profile. Use callgrind, check the cache misses in each case, check the number of instructions executed. Measure the time spent.

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